1. Field of the Invention
The invention provides a device and method for driving a gas displacing piston during the expansion stage of a gas refrigeration cycle. In particular, a tensioning device lifts the piston to a bottom end position during the expansion stage and a compression spring biases the piston to a top end position during other stages of the refrigeration cycle. Alternate embodiments of the invention may utilize pneumatic forces generated by the refrigeration gas to overcome the spring biasing force during the expansion stage to self tune the expansion stage.
2. Description of Related Art
Refrigeration devices based on gas refrigeration cycles are known and commercially available. Such devices include a gas compression unit, or compressor, and a gas volume expansion unit, or expander. The compressor and expander are interconnected by a fluid conduit. The combined internal volume of the compressor, expander and fluid conduit provides a working volume filled with pressurized refrigeration gas. Generally the compressor comprises a compression piston movably supported within a compression cylinder and the expander comprises a gas displacing piston movable supported within an expansion cylinder.
A motive drive force is delivered to the compression piston to reciprocally move the piston over a compression stroke during each refrigeration cycle. Each compression stroke generates a once per cycle peak gas pressure amplitude pulse. The compression stroke forces refrigeration gas through the gas expansion piston and into an expansion space formed in the expander. An expansion stroke moves the gas displacing piston to increase the volume of the gas expansion space approximately synchronously with the occurrence of each peak gas pressure amplitude pulse. The rapid expansion of the gas volume inside the expansion space generates cooling power. The expansion device is said to be tuned when the expansion stroke is initiated synchronously with occurrences of the peak gas pressure amplitude pulses inside the expansion space. A tuned expansion device operates at peak efficiency generating a maximum available cooling power.
Generally, the end of compression stroke minimizes the refrigeration working volume and this condition should correspond with peak pressure pulses of the refrigeration gas throughout the working volume. However in practical systems the peak gas pressure amplitude inside the expansion space may not coincide with the end of the compression stroke such that expansion space pressure amplitude peaks may lead or lag the end of the compression stroke. Moreover, the lead or lag may vary from device to device, may change over time as the device wears and may vary in accordance with operating state of the device, e.g. the lead or lag may be different during the cool down stage. Accordingly many refrigeration devices operated with the expansion device not tuned and therefore inefficiently.
This is especially true in mechanical expander drive systems that mechanically link to the gas displacing piston and apply a continuous driving forces to gas displacing piston over the entire expansion stroke. Such systems are designed with a fixed phase relationship between the compression stroke and the expansion stroke. While mechanical expander drives may provide tuned operating conditions early in the useful life of the device, the tuning tends to degrade as the device wears. Generally mechanical linkage expander drive systems are not self-tuning and can not adapt to changing conditions. However, one advantage of a mechanical expander drive system is that its drive frequency may be varied in order to increase or decrease the cooling power generated with substantially changing the efficiency of the refrigeration device.
Specific examples of commercially available cryocooler configured with mechanical expander drives include the FLIR Systems Inc. models MC-3 and MC-5, manufactured in Billerica Mass., and the Ricor Corporation models K560 and K548 manufactured in Israel. Other examples of integrated cryocooler configurations are disclosed in U.S. Pat. No. 3,742,719 by Lagodmos entitled CRYOGENIC REFRIGERATOR, published on Jul. 3, 1973, and in U.S. Pat. No. 4,858,442 by Stetson entitled MINIATURE INTEGRAL STIRLING CRYOCOOLER, published on Aug. 22, 1989 and commonly assigned with the present application.
Pneumatic drive systems are also known for driving a gas expander piston. Specifically a pneumatic drive system includes a displacer piston movably disposed within a spring volume with the displacer piston rigidly connected to the gas displacing piston by a connecting rod so that the displacer and gas displacing piston move in unison. The spring volume comprises a sealed volume filled with pressurized refrigeration gas in fluid communication with the compressor and the gas pressure inside the spring volume fluctuates between maximum pressure amplitude and minimum pressure amplitude approximately synchronous with the compression stroke. The combined displacer piston and gas displacing piston comprise a piston mass supported for harmonic movement with respect to the spring volume and the gas expansion cylinder. Cycled pneumatic pressure fluctuations in the spring volume provide a harmonic excitation force that drives the movement of the piston mass. Movement of the piston mass is damped by mechanical friction between moving and non-moving surfaces and by fluid drag. As in any single degree of freedom harmonic mass/spring/damping system, the piston mass moves with a natural resonant frequency.
Generally, when a pneumatic expander drive is driven at the natural resonant frequency of the piston mass the expander will self-tune. While this has the advantage that a pneumatically driven expander operates efficiently during steady state operation, there are some disadvantages. In particular, practical expander units have natural frequencies above 50 Hz and devices operated above 50 Hz are audibly noisy. Moreover, during non-steady state operation, e.g. during cool down, the device is usually not tuned and uncontrolled movement of the piston mass is noisier and may cause system damage thereby reducing the reliability of the system.
It is known in pneumatic drive systems to incorporate a mechanical compression spring inside a gas expansion cylinder at one or both ends of the expansion cylinder. Such a compression spring tends to quiet operation and prevent system damage during non-steady state operating periods by absorbing shock energy at one or both ends of the piston travel. However, the use of springs inside the gas expansion cylinder adds dead volume to the expansion cylinder and the dead volume is not usable to generate cooling power. As a result, these systems produce less cooling power per unit of input electrical power to the compressor.
It is also know to incorporate mechanical compression springs inside the spring volume, (see Berry et al. U.S. Pat. No. 5,596,875), to alter the natural frequency of the piston mass. This technique also reduces audible noise and prevents system damage during non-steady state operating periods by absorbing shock energy at one or both ends of the piston travel, but without adding dead volume to the expansion cylinder.
Specific examples of commercially available refrigeration devices configured with pneumatic expander drives include the model LC 1055, offered by CARLETON technologies with headquarters in Orchard Park N.Y., and the model BEI/B512 offered by CMC Electronics of Cincinnati Ohio. Other examples of split refrigeration devices are disclosed in U.S. Pat. No. 5,596,875 by Berry et al., entitled SPLIT STIRLING CYCLE CRYOCOOLER WITH SPRING-ASSISTED EXPANDER, published on Jan. 28, 1997, and in U.S. Pat. No. 4,711,650 by Faria et al. entitled SEAL-LESS CRYOGENIC EXPANDER, published on Dec. 8, 1987.
Generally there is a need in the art to provide an expansion drive that is self-tuning, like a pneumatic drive system, operable at a drive frequency that is below 50 Hz to reduce audible noise and operable over a range of drive frequencies while remaining self-tuning.